This invention relates to a hand-held, motor driven apparatus for sharpening saw blades, particularly large circular saw blades having replaceable teeth as used in commercial lumber cutting operations. While similar saw blade sharpeners can be found in the prior art, there have been no significant advances therein in 40 years. U.S. Pat. Nos. 2,249,743; 2,439,279; 2,651,219 and 3,880,019 each relate to hand-held, motor driven saw blade sharpeners. Of these sharpeners, the one disclosed in U.S. Pat. No. 2,249,743 is the only one which is readily available and commonly used in the industry today. While this saw blade sharpener will perform the job intended, there are several disadvantages with this apparatus as well as the others disclosed in the prior art. For example, all these sharpeners utilize solid grinding wheels, which require frequent dressing and are costly to replace. It is also known that solid grinding wheels will fracture in use on some occasions creating a significant hazard due to the flying grinding wheel pieces. In addition, it can be seen from the disclosures in the above cited patents that none are provided with any means for shielding the operator from the sparks and particles emitted from manual grinding operations, let alone from fractured grinding wheel fragments.
It can further be seen that most of the sharpeners disclosed in the above-cited patents utilize flat grinding surfaces that grind the saw blade tooth in a direction transverse to the cutting direction of the saw tooth. On the other hand, the sharpening apparatus as described below provides a much preferred hollow ground sharpening action as described in U.S. Pat. No. 2,651,219. It is well known that a hollow ground sharpening action, which provides a concave grind perpendicularly away from the saw tooth cutting edge, is far superior. This is so not only because the sharpened tooth need not be dressed to remove burrs and wire edges, but also because the so sharpened saw blade itself is superior not only in providing a better cut, but less energy is required to make a comparable cut. In addition, the hollow ground sharpening action can be effected at a higher RPM having less tendency to burn the saw tooth metal.
In addition to the above there are other disadvantages in the prior art sharpeners as will become apparent when contrasted to the saw blade sharpener of this invention as described below.